1. Field of the Invention
This invention relates to a medical laser; more particularly, to a single laser that provides radiation of two wavelengths, one for cutting biological tissue and the other for cauterizing.
2. Description of the Prior Art
In the past, lasers have been in use as a surgical scalpel, wherein the predominant effects on biological tissue have been thought to be thermal. The laser scalpel can either stem blood flow through photocoagulation or incise tissue through photoablation. Photocoagulation occurs when light absorption produces a temperature rise high enough to denature proteins in the target tissue, a process chemically similar to frying an egg. In the retina, a 10.degree.-20.degree. C. temperature rise will cause photocoagulation; laser photocoagulation is used to sear closed small abnormal blood vessels, such as often form in the retina of diabetics. In the prior art, cutting or photovaporization occurs when light energy is deposited in the tissue, at a rate sufficient to heat the water in the tissue to boiling substantially instantaneously to thereby ablate the tissue.
The laser of choice for a given procedure is dictated by the laser's characteristics as well as the optical properties of the target tissue. Scattering conducts light away from the site of its placement and ultimately results in heating of the adjoining tissue. This can be desirable or undesirable, depending upon the intent of the laser intervention. If the light is at a wavelength that is not strongly absorbed, it penetrates deeply and undergoes extensive scattering, resulting in diffuse heating. This is the case for Nd:YAG (YAG) and argon ion lasers; both are well suited for inducing photocoagulation. If the wavelength is strongly absorbed, there is a rapid rise in temperature at the site where the light strikes and little light penetration or scattering occurs. This results in a well-defined region of photoablation with little lateral spread of heat damage, which is desirable for incisions. CO.sub.2 lasers fall in this category and are used for incision. The CO.sub.2 laser radiation at 10.6 .mu.m is absorbed by water.
Cutting and cauterizing are not quite the dichotomous operations presented above. In many procedures one needs to do both simultaneously. This can be accomplished, albeit not optimally, with a laser operating at a single wavelength, by varying the focal point size and power of laser output. Thus, in using a CO.sub.2 laser to seal a blood vessel, a surgeon may defocus the beam and lower the power. For small vessels, at least, the resulting "edema cuff" may be enough to stop the bleeding. Similarly, in using a YAG laser to cut, a surgeon may focus tightly and increase the laser power. While incisions can be made in that way, there are thermal effects beyond the impact site. While there is essentially no rise in temperature surrounding a CO.sub.2 laser-induced incision, millimeters away from a YAG cut tissue temperature rises to over 200.degree. F. This is obviously undesirable, particularly in surgery on delicate tissue, such as the brain or spinal column.
Since optimal cutting and optimal coagulating are not achieved at the same wavelength, multilaser systems have been proposed by Jako. Up to now cutting has been performed with wavelengths in the far infrared range with coagulating being done with wavelengths in the near infrared or visible range. These systems can deliver a beam from a CO.sub.2 laser, a YAG laser or both (IEEE Spectrum, March, 1985). However, multilaser systems are inherently complex and costly. Further, both lasers deliver light in the infrared wavelength range, CO.sub.2 at 10.6 .mu.m and ND:YAG at 1.06 .mu.m and, as a result the beams cannot be seen. Moreover, the CO.sub.2 laser cannot be used with fiber optics and is thus not suitable for endoscopic cutting applications.
Wengler et al., in German Patent Application No. DD 217 711A, disclosed a surgical laser device that consists of a YAG laser in combination with a second harmonic generating (SHG) crystal. The output of the laser is at 1060 nm in the infrared and is not visible. The SHG crystal can be pivoted into the laser beam path to provide a visible beam for target location, however the second harmonic wavelength at 530 nm cannot cut.
A mechanical device that provides simultaneous cutting and cauterizing was disclosed in U.S. Pat. No. 4,534,347, issued Aug. 13, 1985. The device incorporates in a conventional surgical blade a microwave antenna that emits microwave energy to heat water molecules in tissue cells and cauterize blood at the same time an incision is made.